The present invention relates generally to getter devices and, more particularly, to frittable evaporable getter devices with a high yield of barium, frittable evaporable getter devices with a high yield of barium and reduced activation time, evaporable getter devices with reduced activation time, and an evaporable getter material. Evaporable getter materials are used to maintain a vacuum within the interior of picture tubes for television sets and computer screens. The use of evaporable getter materials within the interior of flat panel displays is also being studied in connection with the development of such displays.
The getter material commonly used in picture tubes is metallic barium. This material is deposited in the form of a thin film on an inner wall of the tube. To form the thin film, an evaporable getter device is introduced in the tube during the manufacturing process. The evaporable getter device typically includes an open metallic container in which a powder compact containing powder of a compound of barium and aluminum, BaAl.sub.4, and powder of nickel, Ni, in a weight ratio of about 1:1 is disposed. In an activation process referred to as "flashing," the device is induction heated by means of a coil situated outside the tube. When the temperature of the powder compact reaches approximately 800.degree. C., the following reaction takes place: EQU BaAl.sub.4 +4Ni.fwdarw.Ba+4NiAl (I).
This reaction is highly exothermic and raises the temperature of the powder compact to about 1,200.degree. C., at which temperature barium evaporation occurs. Barium vapors then sublimate onto the walls of the tube to form the metallic thin film.
Evaporable getter devices are well known in the art. For example, U.S. Pat. No. 5,118,988 to della Porta, which is assigned to SAES Getters, S.p.A., discloses an evaporable getter device in which a number of radial recesses are formed in the free surface of the powder compact to retard heat propagation through the powder compact in a circumferential direction and thereby obtain a controlled barium flash. U.S. Pat. No. 3,558,962 to Reash discloses an evaporable getter device in which a metallic element, e.g., a metallic screen, is at least partially buried in the powder compact to conduct heat to the center thereof and thereby obtain uniform flashing of barium.
The manufacturing processes for both traditional picture tubes and flat panel displays involve the joining of two glass plates in a so-called "frit sealing" operation. In this operation a glass paste having a melting temperature of about 450.degree. C. is melted or softened between the two glass plates in the presence of air. After the frit sealing operation, a getter device may be introduced in traditional picture tubes through the neck provided for housing the electronic gun. In this case, however, the size of the getter device is limited by the neck diameter and precise positioning of the device within the picture tube is difficult. On the other hand, in the case of flat panel displays, it is practically impossible to position the getter device after the frit sealing operation. Consequently, picture tube manufacturers tend to insert the getter device before the frit sealing operation. One drawback with this practice is that the getter device is exposed, at a temperature of about 450.degree. C., to atmospheric gases and the vapors released by the low-melting temperature glass paste during the frit sealing operation. The primary result of such exposure is the oxidation of nickel on the surface of the powder compact. During barium flashing, the thus-formed nickel oxide and aluminum undergo a highly exothermic reaction which cannot be controlled. This may lead to a portion of the powder compact being raised from the bottom of the container, the ejection of fragments of the powder compact from the container, or the partial melting of the container. These problems are detrimental to the proper operation of both the getter device and also the tube as a whole. A more controlled barium evaporation could theoretically be obtained by supplying the device with less power during the flashing operation. This solution would not be acceptable in the picture tube industry, however, because it would increase the evaporation time.
Evaporable getter devices which can withstand frit sealing conditions, i.e., exposure to an oxidizing atmosphere at 450.degree. C. for up to two hours, without suffering from the above-described drawbacks are referred to as being "frittable." Frittable evaporable getter devices are commercially available from SAES Getters S.p.A. of Milan, Italy, the assignee of the subject application. Such devices can be manufactured using conventional technologies provided certain parameters are not exceeded. In particular, the thickness of the powder compact cannot exceed a certain maximum thickness because, at greater thicknesses, the heat generated in the powder compact dissipates slowly, which gives rise to the above-described problems. It has been found empirically that the ratio between the quantity of barium in the device, in mg, and the diameter of the device, in mm, should not be more than about 10. For reasons dictated by the process by which picture tubes are manufactured, the maximum diameter of frittable evaporable getter devices is about 20 mm. Consequently, the maximum quantity of barium that can be evaporated from such devices manufactured in accordance with conventional technologies is about 200 mg. Large picture tubes currently being produced require at least 300 mg of barium, however. As such, conventional frittable evaporable getter devices cannot provide the amount of evaporated barium required for such large picture tubes.
For purposes of the discussion herein, frittable evaporable getter devices capable of evaporating in excess of 200 mg of barium will be referred to as "high yield" devices. Attempts to obtain such high yield devices by resorting to prior solutions which have provided excellent results in the case of non-frittable getter devices have been unsuccessful. For example, when radial recesses are formed in the surface of the powder compact as described in U.S. Pat. No. 5,118,988, the barium evaporation process following the frit sealing operation causes swelling of the powder compact or the ejection of fragments therefrom. Devices formed in accordance with U.S. Pat. No. 3,558,962 are also non-frittable because they suffer from the problems described above, regardless of whether the metallic screen is welded to, or otherwise in contact with, the bottom of the container or is pressed into the free surface of the powder compact.
The production of frittable getter devices without dimensional limits, which are consequently high yield devices, is described in various patents. For example, U.S. Pat. No. 4,127,361 to Hellier et al., which is assigned to SAES Getters S.p.A., discloses evaporable getter devices which can be made frittable by means of a protective organosilane coating. In spite of its efficiency, the process by which this coating is formed is too slow for industrial production.
U.S. Pat. No. 4,342,662 to Kimura et al. discloses a frittable evaporable getter device in which the powder compact is coated with a glass-like film of boron oxide containing up to 7% of silicon oxide. Japanese Patent Publication No. 2-6185 discloses a frittable evaporable getter device in which nickel powder is coated with a film of boron oxide. Both of these devices are difficult to manufacture, however, because such films must have a controlled and reproducible thickness.
In addition to frittability, another important characteristic of evaporable getter devices is activation time, which refers to the time required to evaporate all the barium contained in the device. The activation time, which is also referred to as "total time" or "TT," is measured from the instant the induction heating coil is supplied with power. The TT for conventional getter devices currently being used to manufacture large picture tubes which require at least 300 mg of barium is about 40-45 seconds. This time period corresponds to the slowest step in current production lines for picture tubes. Accordingly, an evaporable getter device with a shorter TT would enable manufacturers to increase the rate at which picture tubes are produced.
A shorter TT theoretically could be obtained either by increasing the power supplied to the coil or by increasing the reactivity of the powders by using powders having smaller particle sizes. However, neither of these approaches is effective in conventional getter devices. Specifically, when the power supplied to the coil is increased, the temperature of the container increases too quickly for homogeneous diffusion of heat into the powder compact to occur, which may lead to melting of the container. When powders having smaller particle sizes are used, an excessive and local increase in the reaction rate between BaAl.sub.4 and Ni occurs which may cause bulging of the powder compact and ejection of fragments of the powder compact from the container.
In view of the foregoing, there is a need for evaporable getter devices which have characteristics such as frittability, high barium yield, and reduced activation time and which do not suffer from the above-described drawbacks of conventional devices.